The package of any electronic device must include contacts for electrically connecting signals, power and ground between the internals of the device and external circuitry. Simple examples of prior-art contacts are the wire leads that protrude from each end of a discrete diode or resistor, or the metal caps on each end of a fuse.
A sophisticated electronic device, such as a microprocessor, may require several hundred contacts. For such devices, surface mount techniques are typically used. The leads of a surface mount device simply contact wires or conductors on the surface of the circuit board, for example, the motherboard of a personal computer system, to which the device is attached. Surface mount leads do not penetrate through the circuit board like a diode or resistor lead typically does, nor do they require a socket as a fuse does. Soldering is typically used to both electronic and mechanically connect surface mount leads to the circuit board.
FIG. 1 illustrates a prior-art integrated circuit that is surface mounted over a circuit board. This integrated circuit includes a silicon (Si) circuit 101. Insulating film 102 coats the underside of silicon circuit 101 to protect and passivate it. Epoxy layer 103 and a silicon cap 104 cover silicon circuit 101. Epoxy layer 103 and silicon cap 104 also cover metal bridge 105.
Metal bridge 105 electrically connects silicon circuit 101 to silicon post 106. Epoxy section 111 mechanically secures silicon circuit 101, metal bridge 105 and silicon post 106. Nickel (Ni) plate 107 covers silicon post 106 and forms a butt-joint with metal bridge 105. Nickel plate 107 is electrically coupled to silicon post 106 and metal bridge 105. Nickel plate 107 provides the integrated circuit with a connection point to external circuitry.
This prior-art contact comprises:
1) metal bridge 105, PA1 2) silicon post 106, PA1 3) nickel plate 107, and PA1 4) epoxy section 111.
As illustrated in FIG. 1, the contact of the integrated circuit has been soldered to circuit board conductor 109 with solder fillet 108. Circuit board conductor 109 has been formed over circuit board substrate 110.
The contact for the integrated circuit illustrated in FIG. 1 provides for various advantages. For example, nickel plate 107 covers the sidewalls of silicon post 106, which helps to strengthen the bonding between the integrated circuit and the circuit board. This is due to the fact that solder can be placed on nickel plate 107 on the sidewalls of silicon post 106 as illustrated in FIG. 1. It also facilitates inspection during surface mount of the integrated circuit to the circuit board. Whether a good mount is made can be easily confirmed by looking at the solder on the sidewalls of silicon post 106.
Furthermore, nickel plate 107 extends over the sidewalls of silicon post 106 and contacts the side of metal bridge 105, forming a butt-joint interface between nickel plate 107 and metal bridge 105. This provides for an electrical contact between circuit board conductor 109 and silicon circuit 101.
The butt-joint interface of the integrated circuit contact of FIG. 1, however, cannot be formed with much certainty or control over its resulting reliability or bonding adhesion between nickel plate 107 and metal bridge 105. There are a number of reasons for this. The physical surface of the side of metal bridge 105 might not be flat enough to ensure a reliable bond at this butt-joint interface. Furthermore, the side of metal bridge 105 is difficult to clean because of its location on the side of the wafer. The bond at this butt-joint interface therefore might be weakened if the side of metal bridge 105 is not flat or has not been thoroughly cleaned.
The formation of this butt-joint interface also limits the materials that can be used for nickel plate 107 and metal bridge 105. This is so because metal bridge 105 and nickel plate 107 can comprise more than one metal layer. The bonding layer of nickel plate 107 then has to be formed so as to bond with each metal layer at the side of metal bridge 105 in order to form an effective contact. Accordingly, the selection of materials that can be used for metal bridge 105 and for the bonding layer of nickel plate 107 is limited.
FIG. 2 shows a prior-art contact that avoids a butt-joint by using a wrap-around flange contact. Silicon circuit 101, insulating film 102, epoxy layer 103, silicon cap 104, metal bridge 105, silicon post 106, solder filet 108, circuit board conductor 109, circuit board substrate 110, and epoxy section 111 are similar to that of the above described butt-joint contact. However, nickel plate 107 and metal bridge 105 have a horizontal flange interface 113. While the wrap around flange avoids the problems associated with a butt-joint, it is still a relatively complex design, requiring a relatively complex series of processing steps and a relatively large amount of wafer area dedicated to contact fabrication.